Light barrier arrangement

Abstract

A light barrier system for monitoring the presence or absence of an obstacle in the monitored area and for simultaneously monitoring itself to detect contamination of the system. A photo-converter emits signals indicative of the light intensity of a light barrier beam and the signal is applied to a first, obstacle switch which is normally open but which closes at a relatively high level of darkness, i.e. when an obstacle is present. A converter signal is also applied to a second, contamination switch which is normally open but which closes at a lesser degree of darkness than the first switch so that both switches can be simultaneously closed. The outputs of the switches when closed are applied to relays to switch the relays between alternative position to indicate (a) the presence of an obstacle in the light curtain area and (b) the contamination of the light curtain system irrespective of whether or not an obstacle is present.

What is claimed is: 
     
       1. A light barrier arrangement capable of monitoring itself for the presence of contamination the light barrier arrangement comprising means for monitoring a beam of light including first and second threshold switches, each threshold switch being adapted to switch from a first state to a second state as the light intensity received by the switch passes through the respective threshold for the switch, the thresholds of the switches being set at respective different levels corresponding to different levels of light flux, and circuit means for detecting when both the switches are in states corresponding to a level of light flux received lying intermediate the levels corresponding to the two thresholds for producing a contamination indication signal. 
     
     
       2. A light barrier arrangement according to claim 1 wherein the first and second switches are adapted to be in the same state on receipt of a level of light intensity intermediate the levels corresponding to their two respective thresholds. 
     
     
       3. A light barrier arrangement according to claim 2 and in which said same state is a non-conducting state. 
     
     
       4. Light barrier arrangement according to claim 1 wherein the circuit means includes light receiving means for monitoring the beam of light, and wherein the first and second threshold switches are connected in parallel with the light receiving means. 
     
     
       5. Light barrier arrangement according to claim 1 wherein the means for monitoring the light beam further comprises a photoelectric converter connected to pass to the threshold switches a signal responsive to the light intensity received. 
     
     
       6. A light barrier arrangement according to claim 3 wherein said circuit means comprises a voltage source, an obstacle indicating relay, a contamination relay, means for connecting said voltage source to the obstacle relay via the first said threshold switch and via a forwardly poled first diode to the contamination indicating relay, means connecting the contamination relay to the voltage source via the second threshold switch and a forwardly poled second diode, the relays each having a pair of associated contacts and means for moving said contacts from a first position to a second position on energization of each respective relay, and means for detecting the positions of the contacts of said contamination indicating relay to produce the contamination indicating signal. 
     
     
       7. Light barrier arrangement in accordance with claim 6 wherein one of the contacts of the contamination relay is disposed as a switch between the first diode and the contamination relay, and including means for insuring that the contamination relay takes longer to disengage than the switching time of the threshold switches. 
     
     
       8. Light barrier arrangement in accordance with claim 7 and in which the last mentioned means comprises means providing the contamination relay with a longer drop out time. 
     
     
       9. Light barrier arrangement in accordance with claim 7 and in which the last mentioned means comprises an RC network connected to the contamination relay, the time constant of said RC network being greater than that of the switching time of said switches. 
     
     
       10. Light barrier arrangement according to claim 11 wherein the switches comprise switch elements switching in the same direction and one of said switch elements is provided with a negator, whereby the switch element provided with the negator switches in the other direction to the other switch comprising the other switch element and no negator. 
     
     
       11. Light barrier arrangement capable of monitoring itself for detecting the presence of contamination, the light barrier arrangement comprising a photoelectric converter for receiving the light intensity prevailing in the light barrier, first and second threshold switches, a variable gain amplifier connected to the photoelectric converter, means for forming a control value for the variable gain amplifier to hold the output potential thereof substantially constant when the light barrier is unobstructed at a level corresponding to an uncontaminated beam, and means for passing the output of the variable gain amplifier to an obstacle indicating device via the first threshold switch, and means for passing the control value to a second contamination indicating device via the second threshold switch. 
     
     
       12. Light barrier arrangement according to claim 11 and in which the means for forming the control value comprises a comparator having a first input connected to sense a value corresponding to the value of the output potential of the variable gain amplifier and a second input connected to a constant reference level. 
     
     
       13. Light barrier arrangement according to claim 12 and further comprising a store including a timing delay element connected to said comparator to receive and store the control value for a time determined by the timing delay element. 
     
     
       14. A light barrier arrangement capable of monitoring itself for the presence of contamination, the light barrier arrangement comprising means for monitoring a beam of light including first and second threshold switches, the threshold switches having different respective thresholds and being adapted to switch in different directions to opposite states when the intensity of light monitored has changed from a value above to a value below their respective thresholds, and from a value below to a value above their respective thresholds, and means for detecting when both the switches are in the same state to produce a contamination indication signal. 
     
     
       15. Light barrier arrangement according to claim 14 wherein the threshold switches are connected in parallel to said means for monitoring the beam of light. 
     
     
       16. A light barrier arrangement according to claim 14 and further comprising a voltage source, means for connecting said voltage source to an obstacle indicating relay via the first threshold switch and via a forwardly poled diode to a second relay, the relays each having a pair of associated contacts, means for moving said contacts from a first position to a second position on energization of each respective relay, and means for detecting the positions of both sets of contacts to produce said contamination indication signal. 
     
     
       17. Light barrier arrangement according to claim 15 and further comprising a variable gain amplifier connected to said photo-electric converter, means for forming the control value for said variable gain amplifier from the output of a comparator, means for supplying to said comparator as input, a value corresponding to the value of the output potential of said variable gain amplifier prevailing during the presence of an uninterrupted light beam, and a constant reference level and means for passing the output of the control amplifier to an obstacle indicating relay via the first threshold switch and to a second contamination indicating relay via said second threshold switch. 
     
     
       18. Light barrier arrangement according to claim 4 wherein the switches comprise switch elements switching in the same direction and one of said switch elements is provided with a negator, whereby the switch element provided with the negator switches in the other direction to the other switch comprising the other switch element and no negator.